The present invention describes an adaptive processor for use on an airborne surveillance aircraft with an electronically scanned antenna. Specifically, this invention improves the detection performance of an airborne surveillance radar system by improving its signal-to-interference plus noise ratio ("SINR").
Adaptive processing for airborne surveillance typically involves linearly combining weighted spatial and temporal samples. The adaptive weights, computed via a signal processor, maximize SINR by incorporating the estimated statistics of the dynamically changing signal environment. Increased SINR improves detection of targets. The radar community generally refers to this type of processor as a space-time adaptive processor ("STAP").
Alternatively, a sequence of linear transformations may be applied to the spatial and temporal samples, in which case the STAP can operate in this linearly transformed domain. For example, applying a two-dimensional Discrete Fourier Transform ("DFT") to the spatial and temporal samples provides a linear transformation from the space-time domain to the angle-Doppler frequency domain. In this case, the STAP operates in the angle-Doppler domain by linearly combining weighted angle and Doppler components. The adaptive weights are thus applied in the frequency domain to maximize SINR. Applying the two-dimensional DFT is equivalent to digital beamforming followed by Doppler processing, thereby transforming the spatial samples to angular frequency and temporal samples to Doppler frequency.
The STAP usually applies to an airborne radar with multiple receive channels, thereby yielding multiply-correlated spatial samples. Several combined radiating elements of an antenna form a subarray, and each of the subarrays comprises a channel. A subarray can be oriented horizontally and vertically with respect to the centerline of the aircraft. Spacing among subarrays oriented horizontally allows spatial sampling and discrimination of signals whose direction of arrival ("DOA") varies in azimuthal angular frequency, also called "azimuth".
On the other hand, spacing among subarrays oriented vertically allows spatial sampling and discrimination of signals whose DOA varies in elevational angular frequency, also called "elevation". In contrast, temporal sampling results from the pulse-Doppler mode of typical airborne surveillance radar. An example of a system that provides azimuthal and elevational spatial samples and temporal radar signal samples is a pulse-Doppler, planar, phased-array antenna.
In a common configuration for surveillance radar, a rotating antenna with a single receive channel electronically scans the antenna's transmit-and-receive beam in elevation. This rotating antenna may comprise slotted waveguides stacked vertically. Using fixed analog hardware, each waveguide aperture combines the received signals into a single channel. This step is called analog beamforming. By varying the electronic phase of the waveguide apertures, the beam can scan in elevation. Implementing STAP on such a system typically requires costly hardware modifications or a complete redesign of the system to create multiple spatial channels for conventional STAP operation.
Thus it is desirable to implement STAP in an airborne surveillance platform having a single-channel, electronically scanned antenna without costly redesign and reconstruction of the radar hardware.